Use of foam patterns (comprised of polystyrene or polyurethane or polyethylene) in the casting of metals has been known for at least the past 20 years. During this period substantially all of such foam patterns have been of simple, unitary shapes. In a few instances, where a complex pattern was necessarily divided into parts to allow the foam parts to be die cast, such parts had to be joined together to complete the pattern.
The splitting of the entire pattern into parts is necessitated by the ability to define, by dies, all the surfaces of the pattern; die molding must allow the dies to be removed along a straight line direction and thus will not allow for undercut or return surfaces. When a pattern requires undercut or return surfaces, the pattern itself is split into at least three pattern parts to obtain the definition of such internally complex surfaces.
It has become the custom to divide such complex patterns along parting planes that pass through a maximum amount of the pattern material and create joining surfaces or joining margins along a substantial portion of the exterior of the pattern. In complex parts, these joining surfaces, or joining margins, are typically thin since the parting plane usually passes through thin walls of the pattern which are to define, in turn, thin cast metal walls.
Foam pattern parts have been joined together prior to insertion in the mold, either by solvent welding, such as illustrated in U.S. Pat. No. 3,675,708 (which unfortunately results in distortion of the welded surfaces), or hot melt adhesive bonding, such as shown in U.S. Pat. No. 4,243,093, which inherently requires the application of hot melt glue along straight, geometrically level lines or surfaces. Commercially available hot melt adhesive bonding equipment (which may be used for mass production joining of such foam pattern parts) is preferably limited to (a) dispensing an extruded liquid bead, or (b) printing the hot melt glue onto flat surfaces or straight margins by kissing a hot melt glue-holding platen against the margins which must receive glue. Since most ot the hot melt adhesive employed by such equipment is parrafin based, it is necessarily heated to a temperature of about 260.degree. F. to make it relatively fluid; such glue condition allows it to run off if deposited onto other than level surfaces or level lines, thereby severely complicating the joining process and detrimentally affecting the quality of the casting.
The integrity of the glue joints thus formed between several foam pattern parts is extremely important since it does affect the quality of the casting. Casting quality is affected because the absence of glue, or improperly applied glue at a joint, enables refractory wash (which is applied to the foam patterns after they have been formed and assembled, and prior to insertion into the metal casting mold) to seep into the glue joint. The presence of this wash at the foam part joining interfaces results in an absence of a thin layer of metal in the final casting at such critical areas, creating particularly small defects that must be repaired or the casting scrapped.
It would be helpful if a method of defining the foam pattern parts could be devised so that much of the externally created glue line on the pattern can be reduced in length allowing most of the glue lines, for joining, to be maintained internally within the pattern and not subject to the problem of refractory wash seepage; defects can be tolerated at these interior stations of the casting.
Another problem exists with respect to automating the assembly of the pattern parts, one of such parts having received the hot melt adhesive; it is difficult to bring the pattern parts together accurately on a consistent basis without some supplementary means to pilot the pattern parts into proper position. In the assembly of a complex pattern, where three or more parts are assembled together, this becomes difficult since an interior part does not have the benefit of mold pins to assist such a union.